External masonry walls

6.1.8Concrete blocks

Concrete blocks shall be capable of supporting intended loads, have appropriate thermal resistance and be resistant to the adverse effects of climate. Issues to be taken into account include:

  1. intended loads
  2. freeze/thaw and sulfate attack
  3. thermal resistance.

Intended loads

Blocks should:

  • comply with BS EN 771 and be used in accordance with BS EN 1996-2
  • not be used where they do not support the required load-bearing capacity of the wall
  • be used in accordance with the manufacturer’s recommendations.

The maximum load-bearing capacity of the wall should not exceed the manufacturer’s recommendations. Other factors may dictate the strength of blocks required in certain circumstances, e.g. sulfate resistance may require blocks of greater strength.

For one and two storey homes, blocks with a minimum compressive strength of 2.9N/mm2 should be adequate.

For three storey homes or those with storey heights over 2.7m, 7.3N/mm2 blocks are required for certain parts of the structure, unless structural design shows that strengths lower than 7.3N/mm2 are adequate.

Freeze/thaw and sulfate attack

Concrete blocks used in the outer leaf without protective cladding or render should:

  • have a compressive strength exceeding 7.3N/mm2
  • have a density exceeding 1,500kg/m3
  • be made with dense aggregate to BS EN 12620, or
  • be lightweight aerated concrete blocks having had their suitability confirmed by the manufacturer.

Where there are sulfates in the ground, concrete blocks should not be used below the DPC unless suitability is confirmed by the block manufacturer. Where this is permissible, the mortar should be sulfate-resisting with a mix suitable for the level of sulfates in the ground.

Thermal resistance

Concrete blocks may have been specified according to thermal performance and strength. Alternative concrete blocks should not be used without the designer’s acceptance.